1 Answer
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You should create a product with a starting price of 1m then they can select a length by the Quantity input and the price can adjust that way. You can also set the product price to $0 and force a minimum order qty of 1m.

Another option is to price it in blocks of 5 or 10 meters. Set options for the product so you're only creating say 100 options and you'll only have to set the price in 100 options generated.